The present invention relates to the field of isocyanate-based polymer foams, and particularly to those exhibiting utility in vacuum thermal insulating panels.
Thermal insulating materials have found wide use, particularly in the area of appliance applications, such as for refrigerators, where performance standards and limitations are frequently set by government agencies. At present the most frequently used materials for many insulation applications are rigid polyurethane foams, which are generally closed-cell foams. Choices of blowing agents used in preparing the foams affect the thermal conductivity, also called the K-factor, and promote conformance with governmental as well as manufacturers' specifications. The blowing agents most frequently used, and also most frequently governmentally regulated, include, for example, chlorofluorocarbon compounds and certain other halogenated compounds. These blowing agents have been generally found effective to allow K-factor values in the range of from 0.1 to 0.3 BTU*in./sq.ft.*hr.*degrees F. (14.4 to 43.2 mW/m degree K), but even lower K-factors will be needed in the future.
For appliance uses one approach to providing thermal insulation has been the use of vacuum insulation panels. Various designs of these panels have been disclosed, using different interior or "core" materials. A common core material is precipitated silica and other inorganic powders, which are processed using various ceramic techniques to provide the appropriate size and shape part. The part is then evacuated to a low pressure in order to decrease the K-factor in comparison with an unevacuated part. These parts often provide good thermal insulation, but their preparation may expose the preparer to environmentally-induced health problems. Another common core material is open-cell polyurethane foam, which can be similarly evacuated to decrease K factor. However, the K-factor of these commercial open-cell polyurethane foams, even upon evacuation, is generally much higher at a given pressure than the K-factor of precipitated silica-filled vacuum panels.
Some researchers have employed aerogels in their attempts to improve product K-factor values. Aerogels are defined as a special class of open-cell foams derived from the supercritical drying of highly cross-linked inorganic or organic gels. These materials have ultrafine cell/pore sizes (less than 1000 A), continuous porosity, high surface area (400-1000 m.sup.2 /g), and a microstructure composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 100 A. This microstructure is responsible for unusual optical, acoustic, thermal and mechanical properties. See, for example, LeMay, J. D., et al., "Low-Density Microcellular Materials", MRS Bulletin, A Publication of the Materials Research Society, Volume XV, Number 12 (December 1990).
For example, U.S. Pat. No. 5,122,291 discloses a thermal insulating material based on a pigment-containing silica aerogel. The method of preparation includes the supercritical processing required for aerogel production, and also flash vaporization of the inert liquid. U.S. Pat. No. 4,966,919 discloses composite foams which include a first foam having pore sizes from about 1 micron to about 30 microns and, incorporated therein, a second foam having pore sizes from about 0.01 micron to about 1.0 micron. Silica aerogel-filled polystyrene emulsion foams are included as suggested materials.
Further discussion of the preparation of ceramic aerogels is provided by, for example, Deshpande, et al., in "Pore Structure Evolution in Silica Gel During Drying/Aging. III. Effects of Surface Tension," in J. Non-Cryst. Solids, 144(1), 32-44 (1992). U.S. Pat. No. 4,667,417 discusses ceramic and organic aerogels which are prepared by drying of organic and inorganic hydrogels. Finally, Pekala, et al., in "Thermal Properties of Organic and Modified Inorganic Aerogels", a preprint available from Lawrence Livermore National Laboratory, calculated the optimum density for a hypothetical air-filled polyurethane aerogel, to find the average pore size needed for R=20 insulation value. Other researchers have disclosed aerogels prepared from melamine-formaldehyde, resorcinol-formaldehyde and a variety of other resin bases.
A problem encountered with many of the disclosed aerogels, whether organic or inorganic, however, is that they are expensive and difficult to prepare, because of their supercritical processing conditions. They also tend to be very fragile, shattering easily, which may present shipping and storage problems and may limit use in some applications. Because of these drawbacks, some researchers have looked at a lower cost alternative employing similar starting materials. When these materials are used to prepare cross-linked inorganic or organic sol-gels which are then dried by solvent evaporation under non-supercritical conditions, rather than supercritical high- or low-temperature extraction, the result is a xerogel. Xerogels are similar to aerogels in that they exhibit extremely fine pores and continuous porosity, but because of the difference in processing conditions tend to be denser and are more easily handled. Their porosity is dependent upon the precursor chemistry, but is generally less than 50 percent.
Because of the problems encountered with previously known means of providing insulation, particularly for appliance applications, many manufacturers are now seeking alternative approaches to providing thermal insulation. Accordingly, it is desirable to develop alternative compositions and methods of preparing insulative materials for these application.